Known containers of, for example, white or colored sheet metal, glass or ceramic are being increasingly replaced by plastic containers. Plastic containers are being used for packaging of fluid substances, for example, beverages, oil, cleaning products, cosmetics, etc. The low weight and lower costs of plastic containers is a factor in deciding on their use. The use of recyclable plastic materials and the favorable overall energy balance in their production also contribute to promoting the acceptance of plastic containers, especially plastic bottles, among consumers.
The production of plastic containers, especially plastic bottles, for example from polyethylene or polypropylene, can take place in extrusion blow molding processes, such as in an extruded tube blowing process. In this connection a plastic tube is extruded from an extruder head, placed in a blow molding tool arrangement, inflated by overpressure via a blow mandrel, cooled and removed from a mold. The extrusion blow molding machines used for this purpose can have at least one extruder for supplying a molten plastic material. The output of the extruder can be connected to an extruder head which has an extrusion die with an annular gap whose width can be controlled via a mandrel which can be axially adjusted relative to the extrusion die. The plastic tube can be extruded continuously or quasicontinuously in one or more layers. The extruded plastic tube can be transferred to a blow molding tool arrangement and can be inflated by overpressure using a blowing mandrel which has been inserted into a mold cavity. Afterwards the plastic container which has been inflated according to the mold cavity can be removed from the mold cavity.
During inflation the plastic tube which has been placed in the mold cavity can be stretched to different degrees depending on the geometry of the plastic container which is to be produced. To ensure that a desired minimum wall thickness is reached at critical locations in this way, for example on edges or corners, in known methods, the wall thickness of the extruded plastic tube has been set overall such that following the blow molding process a sufficiently large wall thickness remained at the more heavily stretched locations on the plastic container. In known methods of production of plastic containers which may not fall below a certain minimum wall thickness in defined regions in their axial extension which runs in a peripheral direction as dictated by design in order to have sufficiently great strength, a total wall thickness of the extruded plastic tube can be oriented to achieving the desired minimum wall thickness. In this way material consumption can be increased in a manner which increases production costs and is wasteful.
To save material, different process versions have been proposed in order to achieve a wall thickness distribution which is as uniform as possible both in a lengthwise and a circumferential direction and to increase the wall thickness of certain critical regions in a controlled manner. For example, DE 199 03 084 discloses providing the discharge nozzle of the extrusion head with an elastically deformable sleeve. The sleeve is pressurized by two actuating cylinders on opposite sides and deformed to a desired extent. An annular gap which is bordered by a mandrel and the sleeve can be changed by a radial deformation of the elastic sleeve. In this way, material can be forced into desired areas on the extruded plastic tube. Alternatively or in addition, the mandrel can have a peripheral region which can be partially adjusted statically or dynamically. This is disclosed, for example, in DE 199 04 199 or DE 199 29 381. These methods and arrangements can be used only for producing relatively large plastic containers, such as for example canisters, plastic barrels, or the like. The actuating cylinders for controlled deformation of the elastic sleeve occupy a relatively large amount of space. Therefore the arrangement for extruder heads with which several plastic tubes which run parallel to one another at the same time are extruded in order to then be inserted into a multiple blow molding tool is not suitable. The arrangement is designed in a container-specific manner. Refitting to another container type can only be done with great difficulty, is labor intensive, time-consuming and expensive.
EP 1 685 943 discloses an extrusion head whose discharge nozzle tapers conically toward the outlet and interacts with an axially adjustable mandrel in order to change a width of an annular gap. The mandrel is equipped with a concentrically arranged plunger which need not necessarily be designed to be rotationally symmetrical and which can be axially moved by a hydraulic actuating mechanism relative to the mandrel. By extending the plunger, the gap width which is fixed by the mandrel and the discharge nozzle, can be additionally tapered if desired. This is designed to displace the material of the plastic tube which is extruded through the annular gap and the material is thus redistributed to the desired extent. The “plunger in mandrel” version can be complex in construction and expensive. This mandrel, due to the plunger embedded in it and the actuating mechanism, has a relatively large outside diameter. Therefore this device can be suitable for producing relatively large plastic containers, such as plastic barrels, plastic tanks or the like. The device is designed for a specific container type and refitting can be time-consuming and expensive.